Two things are clear about the size of the crowd that gathered downtown for Tuesday's inauguration:

First, although some experts described it as high, the official estimate released by the District yesterday is 1.8 million, a figure that would make the gathering the largest ever on the Mall.

Second, from space we look like ants.

Authorities for years have been wary of estimating crowd sizes for events on the Mall, but numbers poured forth yesterday -- including from D.C. Mayor Adrian M. Fenty's office.

In radio interviews, Fenty (D) cited the 1.8 million figure, which eclipses the 1.2 million official tally from the 1965 inauguration of Lyndon Johnson.

Mafara Hobson, Fenty's spokeswoman, described the count as a "collective decision" reached Tuesday inside the city's joint operations command center, where dozens of local and federal agencies were helping to coordinate security and traffic management. She said it was based on information collected by several cameras and individuals on the ground.

"The 1.8 [million] was communicated out to the mayor's office and other agencies," she said.

The National Park Service, which has been barred from making estimates since its count of 400,000 at the 1995 Million Man March drew the threat of a lawsuit, issued a statement yesterday saying it would "not contest" the 1.8 million estimate.

"We do firmly believe that the crowd that was there was the biggest crowd ever, and I think that's what people are most interested in," Park Service spokesman David Barna said. "We don't have anything that suggests there was ever a bigger crowd."

But three experts arrived at lower estimates after viewing a satellite image of the event. Steve Doig, a journalism professor at Arizona State University, estimated that 750,000 people were on the Mall and in the ticketed areas near the Capitol, excluding the parade route and city streets.

There are caveats: The satellite image was taken at 11:19 a.m., 45 minutes before the swearing-in. At the time, spectators were still gathering.

Also, the image does not capture the thousands of people who were sheltering in museums or walking through the Third Street tunnel. Shadows compound the difficulty of counting people on city streets.

"We'll probably always be in the realm of estimates," said Jeffrey Schnapp, head of Stanford University's Humanities Lab and its Crowds Project. "Every view is a partial view, even from a satellite. There are all kinds of obstacles that will give you an undercount. It may be that those you missed amounted to 10,000 or so and the figure is still largely accurate, or it may be that many more were not in the frame."

The photograph was taken from 423 miles up by a camera on the GeoEye-1 satellite as it passed over central West Virginia, according to Mark E. Brender, a spokesman for GeoEye, a satellite and mapping company based in the Dulles area. The camera can see things the size of home plate on a baseball diamond, Brender said.

The Washington Post's analysis of the image concluded that about 1 million people were on the Mall. The analysis did not include the parade route, which was supposed to accommodate 300,000 people at its height, or city streets.

Carl Holmberg, a retired assistant chief of the U.S. Park Police who calculated and reviewed Mall crowd estimates for more than two decades, estimated that there were about 1 million people on the Mall, and about 1.2 million with the parade route included.

"It's the biggest I've seen," he said.

The British defense and security company IHS Jane's announced an estimate of 1.27 million to 1.65 million people, excluding the parade route. That figure, like The Post's, assumed that all 240,000 ticket holders accessed the reserved areas near the Capitol.

Many ticket holders have complained that problems at checkpoints prevented them from entering, and the image shows large knots of people outside several gates.

According to a security official, the D.C. Police Crowd Management Division estimated internally that 1.4 million people were on the Mall at the crowd's peak, another 300,000 to 400,000 on surrounding streets and 72,000 on the parade route.

The satellite image depicts a crowd assembled in giant clusters, mostly between the west front of the Capitol, where the swearing-in took place, and the World War II Memorial. The clustering is thickest at the Capitol ticketed area, around the Washington Monument and in spots along the Mall between the monument and the Capitol, where people appeared to gather around the Jumbotrons broadcasting the inauguration.

The clusters challenged analysts, said Allison Puccioni, a satellite imagery expert who did the Jane's count. "The crowd density was so unique and unexpected," she said.

The estimate of the inaugural crowd from 1965 was made by an assistant police chief for the District who has since died. Some experts have challenged that number. The technology used to count crowds from aerial photos has advanced dramatically over the years.

"If I had to bet, I would say the [Barack] Obama crowd is in fact bigger than those that showed up for [Johnson] or any of the other things," Doig said. "I'm wholly prepared to think it was the largest crowd."